Within the industry, preferably the automobile and domestic machine industry different types of sealing compounds and glue are used, which in floating form are applied onto at least one surface of the workpiece, which must be sealed or joined. The material that is used for usually sealing/gluing contains chemicals and substances, which are directly or indirectly unhealthy for the people through contact or inhalation. Generally, it is required that one uses protective clothing, which makes working difficult. In case of glue in solid form, it is needed that the glue melts and is applied in floating form, which requires that the dosing unit is equipped with heating element, which in turn requires heavier and bulkier apparatus. At sealing, it is very important that the sealing is done appropriate, without leaking parts.
The above work can be done manually, but recently use of robot systems for application and dosage of viscous material of above-mentioned types has begun in a great extent. In such systems, a dosing unit is arranged on a robot arm which moves over the workpiece to be furnished with the viscous material. The robot arm controlled by a computer moves in accordance with programmed instructions and dispenses, for example the glue or the sealing compound by applying strings of material on the contemplated areas.
The amount of the dispensed material, which should not be below or exceed a predetermined amount is also significant. This implies that one must measure and adjust the flow exactly through the dosing unit.
Dosing systems are commercially available where a doser is arranged on a robot arm. Nearby is arranged a docking/pump station, where that viscous material to be dispensed is kept. During the operation, the robot docks the doser in the docking station and fills it with the viscous material. The amount of the material varies in respect of the size of the dosing unit. Subsequently, the robot arm is moved over the workpiece and the material is applied on the same.
The materials applied/dispensed are usually very viscous and some requires frequent heating to be able to handle it at all. The normal value for viscosity is 50.000-500.000 cp. Examples of such material are all types of one component material such as silicon, epoxy, polyurethane, PVC, Butyls, all types of grease as well as different solvent based glues or the like.
Typical application temperatures are between +15.degree. and +150.degree. C., preferably 30.degree., 70.degree., 140.degree. C., where a small temperature deviation results in a significant change of viscosity. Tests of certain material have shown that, for example 2.degree. lower temperature results in 12% lower flow and 5.degree. results in 26% lower flow.
The characteristics of the materials, for example thixotropy, can be changed depending on how the material is handled, e.g. if the material rests a long time in the dosing unit, it becomes more viscous.
When the material to be dispensed is very viscous, such as sealing compounds etc., measuring the flow with conventional flow meters is very difficult and there are very few flow meters which can do so. If the material is treated too much, for example in a gearwheel meter, it will be "grinded into pieces" and thereby show completely other characteristics. This is why it is desired to measure the material flow without contact with the material. Furthermore, the materials wear down the equipment significantly and it is difficult to clean the equipment, which results in frequent calibration and measurement error adjustment, which is done manually.
WO 88/07713 discloses a method and device, mountable on a robot arm, for dispensing material in floating form, which is dosed from a nozzle in an amount which is controlled by means of a measuring valve including a seat and a pin connected to a double-acting piston, which can be moved relative the seat and modulate material flow. A servo actuator connected to a feedback, controls and positions the piston and the pin relative the seat with a control signal, generated with regard to the difference between an operation signal which presents the set flow and the sum of a pair of feedback signals, one of which shows the actual flow amount while the other corresponds to both the relative speed and position of the pin in respect of the seat.
The device uses a so-called position dependent movement feedback and the principle is based on flow limiting with variable area, where the relationship between the adjustment sensitivity increases with increasing area between the valve pin and the seat. The increasing area is obtained by adjusting with small distances between the pin and the seat and in this way obtaining large variables in the flow area. Further, the positions of the double-acting piston and the pin are fed back to a control circuit by means of an electromagnetic inductance circuit. A flow sensor is located at the nozzle which generates a first flow signal, as the first feedback signal to the control system. The first feedback signal is generated by a measured signal based on the pressure drop over the nozzle. Another position dependent feed back signal is generated in respect of the position of the pin and feedback to the control system. The first and second signals form a connection based on the flow-pin opening. The function of the system is based on a certain material with a certain viscosity, whereby a certain distance between the valve pin and the valve seat generates a certain flow and a certain pressure drop over the nozzle of the valve. A problem with this system is that most of the materials which should be dispensed contain filling materials, which wear down and influence the parts that are in the flow path, for example the valve seat. Since the principle of the adjustment in a higher degree depends on the relationship between the seat, pin and the opening gap, a little wear will result in a major flow error. Consequently, the reliability of the system is highly related to the degree of filling material in the material to be dispensed. Furthermore, no discussions are made on the relationship between the actual and set value and how reliable a primary flow as basis for continuous adjustment is obtained. The system does not manage dynamic handling of variations in the flow, for example, because the viscosity of the dispensed material can change as a result of temperature differences or presence of air. Neither the system can handle a continuous flow value during the entire application cycle nor provide the total applied amount of the material during a cycle.